Abrasive articles generally comprise abrasive grains secured within a binder. In the case of a bonded abrasive, the binder serves to bond the abrasive grains together such that they form a shaped mass. Typically, this shaped mass is in the form of a wheel and thus it is commonly referred to as a grinding wheel. In the case of coated abrasives, the binder serves to bond the abrasive grains to a substrate or backing, and the binder may be comprised of make and size coatings. In the case of nonwoven abrasives, the binder serves to bond the abrasive grains to a lofty, open, fibrous substrate.
In many abrasive articles the binder includes a particulate filler. Typically, the binder will comprise between 40 to 70 percent by weight particulate filler. The addition of the filler either increases the toughness and hardness of the binder and/or reduces the cost of the finished article, e.g., by decreasing the amount of binder required. The filler is typically an inorganic particulate material, generally having a particle size less than about 40 micrometers. Examples of common fillers in the abrasive industry include calcium carbonate, calcium oxide, calcium metasilicate, alumina trihydrate, silica, kaolin, quartz, and glass.
There exists a subclass of fillers, referred to as grinding aids, cutting aids, or generically as "active fillers". An active filler is typically a particulate material the addition of which to the binder has a significant affect on the chemical and physical processes of abrading which leads to improved performance. It is believed that active fillers will either 1) decrease the friction between the abrasive grains and the workpiece being abraded, 2) prevent the abrasive grains from "capping", i.e. prevent metal particles from becoming welded to the tops of the abrasive grains, 3) decrease the interface temperature between the abrasive grains and the workpiece, or 4) decrease the required grinding force.
Cryolite (Na.sub.3 AlF.sub.6) is considered by those knowledgeable in the abrasives industry to be an active filler. Cryolite is a particulate material, typically having an average particle size less than about 50 micrometers, usually less than about 20 micrometers.
In general, active fillers are most efficient when used in a dry grinding mode.
The mechanism of wet grinding is much different than dry grinding. Active fillers such as cryolite generally do not function as effectively in the wet grinding mode. For equivalent amounts of stock removal, grinding may be done at increasingly higher forces, eventually leading to dulling of the abrasive article through degradation of the binder and/or abrasive grain pullouts. Abrasive product dulling may also occur as a result of the abrasive grains being worn to the make coating level.
Thus, an unmet need exists in the abrasives art, particularly in the art of wet grinding, for an active filler that provides for a controlled erosion of the abrasive layer during grinding, which results in a continuous exposure of the working abrasive grains, a slower increase in the required grinding force, and prolonged life of the abrasive article.
The following discussion evidences the abrasives industry's interest in fillers and grinding aids.
Assignee's copending application Ser. No. 594,104 describes erodable agglomerates in two forms:
"In one form, the erodable agglomerate can consist essentially of a binder and a grinding aid. In another form, the erodable agglomerate can consist essentially of a grinding aid. . . . In this particular form of the erodable agglomerate, the binder is absent and the grinding aid has a particulate size sufficiently large to form an erodable agglomerate. . . . The binder of the erodable agglomerate can be inorganic or organic . . . typically comprises a resinous or glutinous adhesive . . . . "
Assignee's U.S. Pat. No. 5,078,753 describes similar erodable agglomerates. In each of these references, the erodable agglomerates are preferably made by forming a homogeneous mixture of the grinding aid in the resinous or glutinous adhesive (or by dispersing the grinding aid in a medium), curing the adhesive (or drying the dispersion to form a cake), and mechanically crushing the cured mixture or cake to form the agglomerates using roll crushers or jaw crushers. In agglomerates including binder, the binder can be selected from phenolic resins, polyester resins, and the like.
While the above-mentioned erodable agglomerates have been found to be quite useful in the production of abrasive articles, they do not provide a controllable mechanism to degrade under wet grinding conditions, and their production involves the energy and labor intensive steps of crushing and screening to achieve the desired agglomerate size distribution. It would therefore be desirable to avoid these steps and provide an agglomerate that erodes both under mechanical forces and under wet grinding conditions.
U.S. Pat. No. 4,311,489 (Kressner) describes an agglomerate of fine abrasive grains, such as aluminum oxide abrasive grains, and an inorganic, brittle, matrix formed for example of cryolite. Two methods of forming the agglomerates are disclosed, each having their own disadvantages. In the first method, the abrasive grains and matrix are fused at high temperature (1050.degree.-1100.degree. C.) and then crushed to achieve the desired grain size, two obviously energy and equipment intensive steps. The second method employs a silicate binder containing an active filler such as cryolite. The abrasive grains, cryolite, and a silicate solution (such as potassium silicate, 38% solids) are combined to form a mortar-like mixture which is deposited onto a silicone release substrate, dried in a hot-air oven at about 120.degree. C., and crushed to the desired grain size. This method is also quite energy and equipment intensive.
U.S. Pat. No. 4,381,188 (Waizer et al.) describes a grinding disk including filler pellets or granules consisting of a bonding agent (such as synthetic resin), a matrix, and substances embedded therein, the matrix being made of fine grained fillers resistant to water and air, such as cryolite, and substances selected from active fillers susceptible to water or temperature, e.g., ferric chloride, tin (II) chloride, manganese chloride, embedded in the matrix. The pellets or granules are preferably made in a dry mode by means of rollers having recessed portions. The composition of the pellets is adapted to "guarantee sufficient protection of the hygroscopic or otherwise unstable substances . . . the constituent of the highly hygroscopic substances . . . should not exceed 30-40 percent by volume preferably, they are present in any amount of 10-40 percent by volume" referenced to pellet volume.
Other references of interest describing the use of cryolite, inorganic sulfates, and the like, as fillers in abrasive articles include U.S. Pat. Nos. 2,016,892; 2,308,982; 4,253,850; 4,475,926; 4,609,381; 4,761,163; 4,903,440; and 4,907,376, and European Patent Application 0 061 035 A2 (published Sep. 29, 1982). However, none of the references teaches or suggests the production or use of erodable filler agglomerates.
The presence of active fillers in coatable or moldable mixtures tends to improve the abrading characteristics of abrasive articles incorporating cured versions of same. However, the abrasive industry is always evaluating means to improve the abrading efficiency (i.e., weight of workpiece removed per weight of abrasive article lost) of abrasive articles without unduly increasing their cost.